The long term of our studies is to understand the functional contribution of proteolytic systems to cancer progression. We have focused on the action of membrane type- matrix metalloproteinases (MT-MMPs) known to promote pro-invasive activity through tissue barriers, particularly MT1-MMP (MMP14). We now show a novel interaction between three members of the MT-MMP family, MT1- (MMP14), MT2- (MMP15), and MT3- (MP16) MP (refered here to as MT-MMPs), and the discoidin domain receptors (DDRs), a unique set of receptor tyrosine kinases (RTKs) that are specifically activated in response to collagen, which together with MT-MMPs' collagenolytic activity may orchestrate the proteolytic and adhesive programs during tumor cell dissemination. Preliminary data show a unique functional and specific relationship between MT-MMPs and DDRs that leads to downregulation of collagen-evoked DDR (DDR1 and DDR2) activation in a process that goes beyond receptor cleavage. Indeed, MT-MMPs selectively cleave DDR1 but not DDR2. We also report a gradual loss of DDR1 expression in invasive breast carcinomas suggesting a role for DDR1 in the transition from in situ to invasive carcinoma. Since DDR1 has been implicated in maintenance of normal mammary epithelial function and DDR2 may be associated with mesenchymal- like phenotypes, we posit that a differential regulation of DDRs by MT-MMPs may contribute to the disruption of normal breast tissue architecture and function that leads to malignant cancer. To test this hypothesis we propose thre Specific Aims: 1) To investigate the structural bases, molecular mechanisms, and functional consequences of DDR/MT-MMP interactions, 2) To investigate the expression of DDRs and MT1-MMP in human breast cancer tissues, and 3) To Investigate the DDR/MT1-MMP interplay in experimental models of breast epithelial-matrix interactions. The studies in this application will fill a wide gap of knowledge on our understanding of DDR regulation and function, and wil shed light on how their interactions with MT-MMPs influence cell behavior in normal and malignant breast epithelium. We hope that this new knowledge will help to develop better therapeutic and diagnostic strategies to improve survival in breast cancer patients.